Method of Optimum Controlled Outlet, Impingement Cooling and Sealing of a Heat Shield and a Heat Shield Element

ABSTRACT

There is described a method for cooling and sealing of a heat shield element, comprising a main wall with an inner side, which is restricted by side walls or rims, and an outer side, which can be exposed to a hot fluid, and wherein a coolant is introduced into an impingement region of that heat shield element and an impingement flow of said coolant is directed on a surface area of that inner side through a plurality of impingement holes, effecting an impingement pressure drop. In the method discharge flow is metered through a number of discharge holes through said side wall or rims from the inner side to the outer side of the main wall, generating a discharge pressure drop in series with the impingement pressure drop. The impingement pressure drop and the discharge pressure drop are matched to one another so that a required coolant flow is generated which yields a required predetermined heat-transfer coefficient of the main wall. Discharging coolant into the gaps between side opposing walls of neighbouring heat shield elements only allows for an effective sealing against hot gas pingestion. Furthermore, the invention relates to a heat shield element, preferably to a single chamber or double chamber metallic heat shield element, which can be exposed to hot gases. In particular the heat shield element is suitable for being used in a combustion chamber of a gas turbine installation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2005/055461, filed Oct. 21, 2005 and claims the benefit thereof. The International Application claims the benefits of European application No. 04025338 EP filed Oct. 25, 2004, both of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to a method for cooling a heat shield element comprising a main wall with a cold inner side and a hot outer side, wherein a coolant is introduced into an impingement region of that heat shield element and an impingement flow of said coolant is directed on a surface area of that cold inner side through a plurality of holes for both impingement cooling and flow control.

Furthermore the invention relates to a heat shield element, comprising a main wall with an inner side and an outer side, having an impingement region adjacent to the inner side, said inner side having surface area which can be impinged by a coolant flow introduced through a plurality of impingement holes opposite to said surface area effecting an impingement pressure drop.

BACKGROUND OF INVENTION

Because of the very high temperatures attained in a combustion chamber or in channels through which hot gases flow, it is very important to provide sufficient cooling to the main walls by using as little coolant as possible. In addition hot gas ingestion has to be avoided. For this reason an arrangement of heat shield elements is used and nowadays a few cooling methods are applied for cooling the large surfaces of such arrangements.

Regarding the velocity level and direction from which the coolant flow comes into contact with the area to be cooled there is the pure impingement cooling method—the coolant is blown perpendicularly to the surface and by a vigorous impact heat is transferred, the pure convection cooling method—the coolant is introduced parallel to the area and moves along it, and a combination of the above methods.

Regarding the further downstream use of the coolant two types of circuitries are in use:

“Open cooling”: Here the coolant discharges into the hot gas (simple design but thermodynamically inefficient)

“closed cooling”: After cooling the air is beneficially used, it is ducted to the burners, participating in combustion.

Two kinds of materials can be utilized for the construction of a heat shield element cooled by the methods that were just discussed. On and hand these are high-temperature resistant ceramics. The disadvantage of ceramic materials is their high brittleness. High-temperature Iron-, Chrome-, Nickel- or Cobalt-based metal alloys are the alternative. The thermal conductivity of metals is high, heat extraction is easily possible and the ductile metal is more forgiving the HCF- and LCF-loading. Their operating temperature is limited, however; metals must be cooled sufficiently.

A possible design of the heat shield arrangement used to cool a machine component through which hot gas is passing, especially a combustion chamber of a gas turbine installation, is revealed in WO 98/13645 A1. The arrangement comprises a number of heat shield components with cooling fluid and a hot gas wall to be cooled by that fluid. This heat shield component is composed of two walls—an outer wall which is in contact with the hot gas and a parallel inner wall, so that there is a gap between those two walls. An inlet duct is constructed in such a way, that the cooling fluid is directed towards the inner wall. There it flows through a plurality of apertures, impinges against the outer wall and extends in the direction of this wall. After cooling said wall the fluid flows through an outlet duct running parallel to the inlet duct, leaves the inner room of the heat shield component and is led preferably into the burner of the gas turbine installation.

In EP 1 005 620 B1 a heat shield component is described which is part of a hot gas wall to be cooled. The heat shield arrangement that consists of such heat shield components lines the walls of a hot gas space such as the combustion chamber of a gas turbine. The heat shield component comprises a hollow space; its bottom is exposed to a hot gas, which is attached to a carrier. In the hollow space there is a second hollow body element attached to the same carrier and this element has holes on its bottom. The carrier shows a plurality of inlet channels through which the cooling fluid is fed into the inner space of the hollow body element. The fluid flows through the holes at the bottom of the element, reaches the space between the hollow body element and the heat shield component and impinges the inner side of the bottom of the heat shield component. Then the warmed-up fluid is fed to an outlet channel which opens into the burner of the gas turbine.

Also the EP 1 318 353 A2 discloses heat shields each comprising a liner segment and a support shell for a combustor. The support shell is spaced apart the liner segments to define chambers there between. Impingement cooling holes are arranged in the support shell for establishing impingement cooling of the liner segments from the back. Outlet opening are distributed in the liner segments to enable film cooling of the liner segments.

Another arrangement is known by U.S. Pat. No. 5,396,759, disclosing a heat shield for a bulkhead of an annular combustion chamber.

Different areas of the heat shield are either only impingement cooled or only film cooled or only convective cooled. Spent air is discharged such, that a continuous annular flow is achieved.

Summarized, in all heat shield arrangements, especially those used in the combustion chambers of gas turbines, principally compressed air is branched off from the compressor before entering the combustion chamber and used for cooling the wall of the combustion chamber. The advantage is that at any time there is sufficient air at high pressure which can be utilized to remove the heat from the combustion chamber wall.

The biggest drawback is the loss of combustion air and the burner bypass. Moreover by mixing the cold air with the hot gases in the combustion chamber the temperature level decreases. That causes a reduction of the thermodynamic efficiency and the power output of the gas turbine.

SUMMARY OF INVENTION

An object of the present invention is to introduce a method that reduces the overall air consumption for cooling and sealing, especially in the case of open cooling, and thus providing more air for combustion.

Another object of the invention is to provide a heat shield design which utilizes that cooling method.

According to the first object of the invention a flexible method is provided by a heat shield element, comprising a main wall with an inner side, which is restricted by side walls, and an outer side, which can be exposed to a hot fluid, and wherein a coolant is introduced into an impingement region of that heat shield element and an impingement flow of said coolant is directed on a surface area of that inner side through a plurality of impingement holes, causing an impingement pressure drop, wherein after impingement the coolant flow cools the main wall convectively by flowing along the inner side while the coolant converts into a discharge flow, which is drained through a number of discharge holes through said side wall from the inner side to the outer side of the side wall, causing a discharge pressure drop in series with the impingement pressure drop. The impingement pressure drop and the discharge pressure drop are tailored such as to provide a locally required mass flow, which guarantees the predetermined varying heat transfer coefficients on the cold side.

This can be done by using the so-called controlled outlet flow scheme of the cooling air, i.e. one can match the holes through which the cooling air is introduced (impingement holes) with the holes through which the air is drained out of the side walls (or rims) of the heat shield element (discharge holes). Thus tailoring the in-series flow resistances allows two important things:

Adjust safely a defined mass flow for effective impingement cooling, i.e. minimize air consumption.

Prevent effectively any hot gas ingestion into cold structures by discharging into the gaps only, which gaps are built by neighbouring heat shield elements lying apart to each other.

If the static pressure outside the heat shield element after the compressor is P₀ and the static pressure inside the heat shield element is P₁ then the impingement pressure drop is ΔP_(I)=P₀−P₁. Further, if the static pressure at the side of the hot gases is P₂, then the discharge pressure drop is ΔP_(D)=P₁−P₂. The relation among the different pressures is P₀>P₁>P₂. The statement that the impingement pressure drop and the discharge pressure drop are in series means that the overall pressure drop ΔP is: ΔP=ΔP _(I) +ΔP _(D) =P ₀ −P ₂

According to the above equation the overall pressure drop depends only on the initial and final pressure values P₀ and P₂ and doesn't depend on the pressure in the heat shield element is P₁. The “free” pressure level P₁ allows adjusting a required impingement jet velocity level (i.e. heat transfer coefficient). If the impingement pressure drop is increases then the discharge pressure drop is decreases but for a prescribed overall pressure drop ΔP their summation remains always constant and is equal to difference of the pressure values after the compressor and at the hot gas side.

According to a preferred aspect of the method the matching of the impingement pressure drop and the discharge pressure drop includes adjusting the size (hole diameter D_(I),D_(D)) and numbers (N_(I),N_(D)) of both types of holes.

The impingement holes are preferably introduced in a symmetrical order and the distance between every two holes in the pattern is X_(I) for the impingement flow. A hole pattern for the impingement holes can be characterized by taking the ratio between X_(I) and the diameter of the impingement holes D_(I).

According to another preferred aspect of the method a partial impingement flow is directed to a first surface area and another partial impingement flow is directed to a second surface area which is separated from the first surface area. This denotes the application of two sub-regions separated by a dividing wall inside the heat shield element. Varying the pattern of the impingement holes and the discharge hole area of both sub-regions provides different heat transfer coefficients on the first and the second surface area. This design is particularly suitable for an environment where the hot side h.t.c.-gradient is high.

In another preferred development of the invention the relative discharge pressure drop ΔP_(D)/ΔP is to adjust on the required heat transfer rate on the cold side e.g. inner side of the main wall. Where a large heat transfer rate is required the relative discharge pressure drop ΔP_(D)/ΔP at least has to be 70%, for a medium heat transfer rate the relative discharge pressure drop ΔP_(D)/ΔP has to be at least 90% und for a small heat transfer rate the relative discharge pressure drop ΔP_(D)/ΔP has to be at least 97% of the overall pressure drop. This tayloring of the relative discharge pressure ΔP_(D)/ΔP allows to adjust the varying required heat transfer coefficient across the inner surface of the main wall.

One more preferred aspect of the method is that the heat extraction is enhanced also by enlarging the surface area and by generating vortices close to the surface. This can be achieved by surface turbulation and/or by cooling fins and/or stiffening ribs and/or a dimple field positioned along said surface area. Stiffening ribs in turn allow lower main wall thicknesses, i.e. higher heat extraction.

The method of the present invention can be used for a combustion chamber, in particular an annular combustion chamber of a gas turbine. Moreover air can be taken from an air compressor as coolant.

According to the second object of the invention a heat shield element eligible for cooling by means of that method is a heat shield element, comprising a main wall with an inner side and an outer side, which can be exposed to a hot fluid, and having an impingement region adjacent to said inner side, said inner side having a surface area which can be impinged by a coolant flow introduced through a plurality of impingement holes opposite to said surface area effecting an impingement pressure drop, wherein said heat shield element possesses a number of discharge holes along its side wall for draining the coolant through said discharge holes from the inner side to the outer side of the wall, effecting a discharge pressure drop in series with the impingement pressure drop, and wherein the impingement holes and the discharge holes are matched to one another in such a way, that a required jet velocity coolant flow is obtained which yields a required predetermined heat-transfer coefficient α_(R) for the main wall.

The advantages of the heat shield element are implied also from the advantages of the method described above.

Preferred aspects of the heat shield element are the minimization of air consumption and the prevention of hot gas ingestion:

One aspect is the control of local cooling (h.t.c.) via jet velocity/mass flow and P₁ by matching in-series resistances towards ΔP_(I)+ΔPD (pressure drops): Taylor D_(I), N_(I) with D_(D),N_(D).

Another preferred aspect is, that a heat shield element may be constructed as a double-chamber heat shield element where the impingement region is split in two sub-regions or chambers separated by a dividing wall. Said sub-regions may have different impingement hole patterns in order to provide different heat-transfer coefficients when coolant is introduced into the heat shield element. As mentioned above this construction is favorable when the hot side h.t.c. gradient along the heat shield element is high, i.e. that the thermal load of the main wall is strongly differing in the two sub-regions. By means of this design air consumption is further reduced.

The single chamber design gives best results for small to medium heat shield sizes preferably ca 180×180 mm to 240×240 mm; the double-chamber construction for medium size heat shield elements with ca. 240×240 mm to 300×300.

Another way to improve the heat transfer on the cold side of the heat shield element can be done by enlarging the inner surface of the main wall. Surface-increasing elements comprise for instance cooling fins, stiffening ribs, dimples or riblets or combinations of such elements positioned on said inner side of the heat shield element. The surface is enlarged typically in the order of more than 50%, up to about 150% compared to a smooth surface with no surface-increasing elements. In addition riblets, dimples etc. generate vortices close to the surface, thus enhancing the h.t.c.-level even more.

One more preferred aspect is that said heat shield element is made of a metal, in particular of a high temperature resistant metal of high thermal conductivity. As aforementioned the heat shield element can be utilized for cooling a combustion chamber, in particular an annular combustion chamber of a gas turbine. The used coolant can be air, taken from the compressor of a gas turbine.

The proposed method and the heat shield element have a number of additional advantages, which will be briefly discussed below:

The main advantage of the invention is, that by applying the controlled outflow flow scheme the consumption of compressor air extracted for cooling purposes can be reduced by a factor of more or less 2, compared to current practices. For example in a typical conventional cooling scheme about 10% air from the compressor inlet air is required, whereas with the new cooling scheme of the invention the cooling air consumption can be significantly reduced to about 5-6%. Therefore, this directly leads to higher power and efficiency of the gas turbine, because more compressor air can be used for the combustion process.

This large potential suggests using metal heat shield elements being more ductile and forgiving than for example ceramic heat shield elements. The overall new design is simple and robust and very little changes have to be made to current conventional metal heat shields in operation. This new technology implies a high upgrade potential of the state-of-the-art.

The design procedure can be easily applied to other impingement applications as well, for instance aircraft turbine engines, other types of combustion chambers, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures below display:

FIG. 1 a gas turbine installation with an annular combustion chamber with an axial cross section,

FIG. 2 a sectional view of a part of the combustion chamber wall of FIG. 1, showing two heat shields fastened to the carrier,

FIG. 3 a view of the double-chamber heat shield element with discharge holes along the side walls shown in FIG. 2,

FIG. 4 a partial cross section through the main wall of a heat shield close to its single fastening bolt,

FIG. 5 a top view of the part of FIG. 4 showing different means of further cooling enhancements: Turbulation and surface enlargement, dimpling, stiffening ribs, reduced main wall thicknesses, local impingement jets,

FIG. 6 a cross section through a dimpled main wall of FIG. 5 showing surface enlargement and turbulation.

DETAILED DESCRIPTION OF INVENTION

Identical reference indications have the same meaning in the figures.

In the following an embodiment of a heat shield component in a gas turbine is depicted and explained with the accompanying drawings. The proposed designs principally do not necessarily need seals between the combustion chamber carrier wall 11 and the heat shield element 13.

In FIG. 1 a longitudinal cross section of a gas turbine installation is presented. The gas turbine installation 1 includes a compressor 3 for combustion air, a combustion chamber 5 with a burner 7 for liquid or gaseous fuel and a turbine 9 to drive the compressor 3 and a generator which is not shown in this figure. All components are lined on a common shaft along the axis A. In the compressor 3 combustion air L is compressed. This compressed air is fed into a number of burners 7, which are arranged on a circle around the annular combustion chamber 5. Fuel, which is not shown in the drawing, is mixed with a large part of the compressed air and after combustion a flow of hot gases is formed which drives the turbine 9. A smaller amount of the compressed air K is used to cool and seal the combustion chamber carrier wall 11.

In FIG. 2 an arrangement of two heat shield elements 13,131 attached to the combustion chamber carrier wall 11 is drawn. The heat shield element 13 is designed as a single chamber heat shield element and the heat shield element 131 is designed as a double-chamber heat shield element, and the indications a and b refer respectively to the two chambers of the double-chamber heat shield element 131. Opposite to the combustion chamber carrier wall 11 is the heat shield element 13 comprising a main wall 15 with its inner side 151, and its outer side 153 in contact with the hot gases. Attributed to the heat shield elements 13, 131 there is a plurality of impingement holes 21 leading to an impingement region 17, or sub-regions 17 a and 17 b in the case of the double-chamber design. The impingement holes 21 are formed within the combustion chamber carrier wall 11 in this embodiment. They may alternatively be part of the heat shield element 13, 131 or realized as a suitable impingement device placed in the impingement region 17, 17 a, 17 b. On the opposite side of the impingement holes 21 there is a surface area 23, 23 a, 23 b on the inner side 151 of the main wall 15. The discharge holes 27 are only placed to the side walls 14 or rims. The static pressure downstream the compressor 3 is P₀, the pressure inside the heat shield element 13 is P₁ and outside the heat shield element 13 at the side of the hot gases P₂. The cooling air K is led into the impingement holes 21 where it enters the impingement region 17 or sub-regions 17 a and 17 b with a velocity depending on the impingement pressure drop. There it comes into contact with the surface area 23, 23 a or 23 b and transports the heat from the main wall 15. After impingement the coolant K flows in direction to the side walls 14 along the inner side 151 of the main wall 15 while cooling the main wall convective (FIG. 6). Reaching the side walls 14 the discharge flow 25 leaves the heat shield elements 13, 131 through the discharge holes 27 arranged in the rims or side walls 14 and flows straight to the hot gases at the outer side 153 of the main wall 15. Passing through the impingement holes 21 the pressure of the coolant K decreases due to the throttle effect of the narrow impingement holes 21 and the pressure drop there is ΔP_(I). Likewise, the discharge holes 27 result once again in a pressure drop ΔP_(D) which is in series with the pressure drop ΔP_(I). The adjustment of the pressure drops ΔP, and ΔP_(D) characterizes the jet velocities 19, 19 a and 19 b. In turn they define the heat transfer coefficients a, α₁ and α₂ on the different surface areas 23, 23 a and 23 b. The gap between opposing side walls 14 of neighbouring heat shields 13 and 131 is sealed effectively against hot gas ingestion by discharging the whole amount of coolant K, which cools the heat shield element, distributed through all discharge holes 27.

FIG. 3 reveals a view of the double-chamber heat shield element 131 shown in FIG. 2. The heat shield element main wall 15 is displayed with the projection of the impingement holes 21 a and 21 b on it and the dividing wall 29. On the side walls 14 of the double-chamber heat shield element 131 the discharge holes 33 a and 33 b are depicted. The impingement hole patterns 31, 31 a and 31 b characterized by a diameter D₁ and distance between one another X_(I). The originated heat transfer coefficients in both parts of the double-chamber heat shield element 131 are α₁ and α₂. The heat shield element 131 can be firmly attached to the combustion chamber carrier wall 11 via a single central bolt.

In FIG. 4 a partial cross section through a singe-chamber heat shield element 13 is presented comprising also the fastening bolt 35. A stiffening rib 37 is positioned along the main wall 15. The coolant flow K enters the impingement holes 21 and reaches the impingement region 17 as an impingement flow 19, it is directed along the stiffening rib 37 and leaves the inside of the heat shield element 13 through discharge holes which are not depicted in this drawing.

FIG. 5 discloses the top view A-A of a quarter of the heat shield element 13 in FIG. 4 showing different possibilities for cooling enhancements. Impingement holes 21A, 21B, 21C of three sizes are projected to the main wall 15. With regard to the center defined by the fastening bolt 35 a first group of impingement holes 21A is located on a determined inner radius Ri. A second group of impingement holes 21B is located on a determined position outside the inner radius Ri. A third group of impingement holes 21C is located close to the side walls 14 of the heat shield element 13 at a distance from the impingement holes 21A, 21B. The discharge flows 25 through the discharge holes 27 are also revealed in this figure. Two possible means for cooling improvement are displayed—stiffening ribs 37 (to enable thin main walls 15) stretching from the inner side wall 14 around the bolt 35 and the outer side walls 14 of the heat shield element 13 and a plurality of dimples 39 located on the main wall 15. The impingement flow entering perpendicularly or inclined to the main wall 15 cools said main wall 15, flows around and over the surface-increasing elements in the direction of the side wall 14 and discharge holes 27 where it leaves the heat shield element 13. The purpose of the turbulation and surface-increasing elements positioned on the main wall 15 is, that the surface of said main wall 15 is increased and vortices are generated, thus the heat transferred between the main wall 15 and the coolant K is raised.

In example the impingement holes within in radius Ri establishing an impingement pressure drop ΔP_(I) about 70% of the overall pressure drop ΔP an outside the radius Ri about 90% of the overall pressure ΔP. This leads to an impingement cooling varying over the area which is to cool while establishing an sufficient coolant flow speed for the subsequent convective cooling of the main wall 15.

The effect of the dimples 39 positioned on the inner side 151 of the main wall 15 of a heat shield element 13 is shown in a cross-sectional view along the cross section in FIG. 6. The drawing shows the main wall 15 of a heat shield element 13 with a number of dimples 39 formed to the inner side 151 of the main wall 15. The coolant flow K entering the region 17 impinges against the inner side 151 and thus carries away the heat of the main wall 15. Due to the dimples 39 placed on that inner side 151 local vortices 41 occur which enhance the turbulent character of the flow and consequently the heat transfer from the main wall 15. The dimples are preferably spherical cavities into the main wall 15 with a predetermined radius and depth. 

1.-26. (canceled)
 27. A method for cooling a heat shield element attached to a carrier with a plurality of impingement holes, the heat shield element having a main wall with an inner side restricted by a side wall having a number of discharge holes, and an outer side, comprising: exposing the outer side to a hot fluid; introducing a coolant into an impingement region of that heat shield element; directing an impingement flow of the coolant on the inner side through the impingement holes, causing an impingement pressure drop; flowing the coolant along the inner side after impingement to convectively cool the main wall; discharging the coolant through the discharge holes from the inner side to the outer side of the side wall, causing gap sealing and a discharge pressure drop in series with the impingement pressure drop; matching the impingement pressure drop and the discharge pressure drop to one another to generate a required minimum coolant flow which yields a predetermined heat-transfer coefficient of the main wall; and matching the discharge pressure drop to get at least 70% of the overall pressure drop to achieve a first heat transfer coefficient on the inner side of the main wall.
 28. The method according to claim 27, further comprising matching the discharge pressure drop getting at least 90% of the overall pressure drop to achieve a second heat transfer coefficient on the inner side of the main wall, wherein the first heat transfer coefficient is greater than the second heat transfer coefficient.
 29. The method according to claim 28, further comprising matching the discharge pressure drop getting at least 97% of the overall pressure drop to achieve a third heat transfer coefficient on the inner side of the main wall, wherein the second heat transfer coefficient is greater than the third heat transfer coefficient.
 30. The method according to claim 27, further comprising matching the cross sections of the impingement holes and the discharge holes to match the impingement pressure drop and the discharge pressure drop.
 31. The method according to claim 30, further comprising matching the number of the impingement holes and the discharge holes to match the impingement pressure drop and the discharge pressure drop.
 32. The method according to claim 31, further comprising matching the location of the impingement holes and the discharge holes to match the impingement pressure drop and the discharge pressure drop.
 33. The method according to claim 27, further comprising: directing a first impingement flow on a first surface area, and directing a second impingement flow on a second surface area, wherein the second surface area is separated from the first surface area.
 34. A method according to claim 27, further comprising taking air as the coolant from an air compressor of a gas turbine having a combustion chamber having the carrier to which the shield element is attached.
 35. A cooling system, comprising: a heat shield element having: a main wall with an inner side and an outer side exposed to a hot fluid, side walls having a plurality of discharge holes, and an impingement region adjacent to the inner side; and a carrier with a plurality of impingement holes opposite to the inner side, wherein the heat shield element is attached to the carrier, wherein the inner side is impinged by a coolant flow introduced through the impingement holes to effect an impingement pressure drop, wherein the coolant drains through the discharge holes to effect a discharge pressure drop in series with the impingement pressure drop, and wherein the impingement holes and the discharge holes are matched to one another to effect a discharge pressure drop of at least 70% of the overall pressure drop to achieve a first heat transfer coefficient on the inner side of the main wall.
 36. The cooling system according to claim 35, wherein the impingement holes and the discharge holes are matched to one another to effect a discharge pressure drop of at least 90% of the overall pressure drop to achieve a second heat transfer coefficient on the inner side of the main wall, wherein the first heat transfer coefficient is greater than the second heat transfer coefficient.
 37. The cooling system according to claim 36, wherein the impingement holes and the discharge holes are matched to one another to effect a discharge pressure drop of at least 97% of the overall pressure drop to achieve a third heat transfer coefficient on the inner side of the main wall, wherein the second heat transfer coefficient is greater than the third heat transfer coefficient.
 38. The cooling system according to claim 35, wherein the cross-section of the impingement holes and the discharge holes are adjusted to match the impingement pressure drop and the discharge pressure drop.
 39. The cooling system according to claim 38, wherein the number of the impingement holes and the discharge holes are adjusted to match the impingement pressure drop and the discharge pressure drop.
 40. The cooling system according to claim 39, wherein the location of the impingement holes and the discharge holes are adjusted to effect matching the impingement pressure drop and the discharge pressure drop.
 41. The cooling system according to 35, further comprising a double-chamber heat shield element having a dividing wall, wherein the impingement region is split in two sub-regions by the dividing wall.
 42. The cooling system according to claim 41, wherein the dividing wall is attached to the carrier.
 43. The cooling system according to claim 42, wherein the sub-regions comprise different impingement hole patterns in order to provide different heat-transfer coefficients.
 44. The cooling system according to claim 41, wherein the impingement holes of the first sub-region comprise larger hole diameters than that of the impingement holes of the second sub-region to reduce the heat-transfer coefficient in the first sub-region relative to the second sub-region.
 45. The cooling system according to claim 41, further comprising surface-increasing elements to create a higher heat-transfer coefficient resulting in a higher heat extraction, whereby the inner surface of the main wall is enlarged wherein the surface-increasing element is selected from the group consisting of: cooling fins, stiffening ribs, riblets, dimples and combinations thereof.
 46. The cooling system according to claim 35, further comprising a plurality of heat shield elements; wherein at least two heat shield elements are arranged adjacent to each other; wherein the side walls of the heat shield elements form a gap effectively sealed against hot gas ingestion by discharging the coolant flow through discharge holes.
 47. Combustion chamber with a cooling system, comprising: a heat shield element having: a main wall with an inner side and an outer side exposed to a hot fluid, side walls having a plurality of discharge holes, and an impingement region adjacent to the inner side; and a carrier with a plurality of impingement holes opposite to the inner side, wherein the heat shield element is attached to the carrier, wherein the inner side is impinged by a coolant flow introduced through the impingement holes to effect an impingement pressure drop, wherein the coolant drains through the discharge holes to effect a discharge pressure drop in series with the impingement pressure drop, and wherein the impingement holes and the discharge holes are matched to one another to effect a discharge pressure drop of at least 70% of the overall pressure drop to achieve a first heat transfer coefficient on the inner side of the main wall. 